The present invention relates to a circuit for driving a second order harmonic generator to improve a phenomenon of unstable optical output caused by a nonlinear optical element.
In an optical recording method in which data is recorded and/or reproduced using a laser beam, an infrared (e.g., 830 nm) semiconductor laser diode is used to generate a beam to perform recording and reproduction operations. High density recording/reproducing, however, requires a laser diode, because a focused 830 nm laser beam spot has a focal diameter greater than 830 nm. A, laser optical source having a wavelength shorter than 830 nm can reduce the focal diameter and thus increase recording density.
Examples of a short-wavelength laser optical sources include a helium-neon laser and an argon laser. These types of lasers, however, are not suitable for consumer purposes due to their bulkiness, high power consumption, and difficulty in handling.
A second harmonic generator generates a 1064 nm laser optical source, by pumping a Nd:YAG using a relatively small 809 nm semiconductor laser which can be easily handled, and provides a second harmonic (a 532 nm laser beam which corresponds to half the wavelength of the 1064 nm laser beam) using a nonlinear element such as KNbO.sub.3 or potassium titanyl phosphate (KTiOPO.sub.4 : KTP).
Since a second harmonic generator adopts the nonlinear optical element, which is very sensitive to temperature, the optical output maybe unstable. Thus, the temperature of the nonlinear its optical element should be precisely controlled for a constant lasers output.
The structure of a conventional second harmonic generator will first be described below.
An 809 nm laser beam output from the laser diode of a second harmonic generator passes through a first mirror and excites a Nd:YAG located in a resonator, to generate a lased 1064 nm fundamental wave. Here, a Brewster plate and a nonlinear optical element are situated between the Nd:YAG and a second mirror, and the second harmonic wave (532 nm) is generated from the nonlinear optical element.
The beam proceeding toward the second mirror includes both beam components of 1064 nm and 532 nm wavelengths. Since the second mirror has a high reflection ratio with respect to the fundamental wave, the 1064 nm beam is reflected toward the first mirror and only the 532 nm beam is output. Here, a plate whose transmission-to-reflection ratio is 97:3 is used for inputting a portion of the output harmonic beam to an optical detector via a beam splitter. The optical element is used to output a stable second harmonic wave.
When the 532 nm laser beam (second harmonic) reaches the optical detector, a photo-electric conversion occurs, to thereby convert the received beam into a current signal in proportion to the input beam intensity. The current signal is input to a driving circuit for a second harmonic generator. A control signal output from the second harmonic generator is input to a thermo-electric cooler, to thereby control the temperature of the nonlinear optical element.
FIG. 1 is a block diagram of a circuit for driving a conventional second harmonic generator. The structure and operation of that circuit will now be described.
The illustrated circuit for driving a second harmonic generator is composed of an optical detector 1, an amplifier 3, second harmonic optical output setting means 2, a comparator 4, an integration coefficient setting means 5, an integrator 6, a voltage-to-current converter 7 functioning as a driver, and a thermoelectric cooler 8.
The current generated from optical detector 1, corresponding to an optical output from a nonlinear optical element, of a second harmonic generator is input to amplifier 3 to stabilize the optical output. The current signal is converted into a voltage signal by amplifier 3 and input, which comparator 4 to compares the resulting with a set voltage from second harmonic optical output setting means 2. The output from comparator 4 becomes an optical output error signal, the value of which is integrated in integrator 6. The integrated voltage is used to control voltage-to-current converter 7 and drive thermo electric cooler 8, to thereby control the second harmonic generator's temperature of the nonlinear optical element.
FIG. 2 shows the optical output characteristics of a second harmonic generator according to its temperature of the nonlinear optical element at an initial operation state (A) of the second harmonic generator and at a stable state (B) which may be reached, after several minutes. As shown in FIG. 2, the output is higher after stabilization.
Here, as shown in FIG. 2, the nonlinear optical element has a plurality of optical output peaks, according to temperature. An optical output error signal is used to perform negative feedback until the actual optical output value is equal to the desired (set) optical output value. The optical output error signal is applied to thermo-electric cooler 8, to lower the temperature of the nonlinear optical element.
In FIG. 3, the temperature of a nonlinear optical element is cooled from an initial temperature T.sub.1 to a lower temperature T.sub.2 which causes the second harmonic optical output value to reach a set range of optical output values, i.e., to become stabilized. This is done by performing negative feedback. After cooling the nonlinear optical element to the lower temperature T.sub.2, the optical output increases beyond the set range of optical output values between P.sub.0 to P.sub.1, at which point it is decreased. Once the temperature converges set range of output a values, the temperature is held steady. When the optical output varies, the temperature of the nonlinear optical element is adjusted to generate an optical output within the set range.
The second harmonic output characteristics vary according to the temperature of the second harmonic generator, with respect the input optical amount, and across the spectrum.
When the temperature of nonlinear optical element is slowly adjusted from T.sub.1 to T.sub.2 before the second harmonic generator is stabilized, so as to generate an actual optical output within the set range of optical outputs, the actual temperature T.sub.2 of the nonlinear optical element does not exactly reach T.sub.2 and generally overshoots T.sub.2 a peak temperature T.sub.3. When the overshoot optical output value is below P.sub.1 which represents the output characteristic of the nonlinear optical element upon initial operation, the optical output value is stabilized at the set optical output value, to thereby obtain the optical output characteristic curve A shown in FIG. 3.
However, when the overshoot optical output value is over peak P.sub.1, the optical output value is unstable and an intended optical output value cannot be obtained.